A Borough Park mother watched in horror yesterday as her son was run over and killed by a school bus on his second birthday.

The bus was stopped at 1528 49th St. in Brooklyn when the toddler’s mother boarded to retrieve an older son and give the bus driver a Purim gift. At the same time, little Mordechai Gancz ran in front of the vehicle, cops said.

After the mom got off the bus, the driver pulled ahead – running over the 2-year-old, who police said was dead at the scene.

The bus continued to the corner of 49th Street and 15th Avenue before the driver even realized what happened and stopped.

“I saw some commotion, and then I heard a lady yelling,” said Henry Rubenstein, one of the first at the scene. “The mother was hysterical. Someone called 911.”

An ambulance was soon on the scene, “attending the child, and one worker was attending the mother,” Rubenstein said.

Borough Park’s tight-knit Orthodox Jewish community rallied around Mordechai’s mom, who was not identified by police.

Neighbors said the tragedy casts a pall over the Jewish Purim holiday, which starts today at sundown.

“She did a big mitzvah [good deed]. She wanted to give a present to the bus driver, and she did not see her child run after her,” said Nill Rychik, a teacher who lives across the street from the Gancz family.

“This is a total shock. I’m shaking. I’m devastated,” said Rychik. “This just shows us from moment to moment, you don’t know what can happen.”

“We are very hurt and sad this happened,” said Sammy Gross, 28.

“She [the mother] is holding up OK,” Gross said. “It didn’t get to her yet, that’s what I think.”

No charges are planned against the driver, identified by cops as Mordechai Lax, 34. Police said Lax was properly licensed. The bus serves a Hasidic school, but police did not name its owner.

“We’re treating this as a tragic accident,” said Capt. Robert Johnson of the 66th Precinct.

A police officer on bicycle patrol was injured responding to the accident when he was struck by an ambulance rushing to the scene, said cops.

The officer, who was not identified, injured his left knee. He was in stable condition at Maimonides Hospital.